It has long been known that naturally occurring hydrocarbons can be cracked at high temperatures to produce valuable olefinic materials, such as ethylene and propylene.
The growth in the propylene based plastics market relative to the ethylene based plastics market has made it desirable to improve the propylene yield when cracking hydrocarbons to olefins.
In addition, higher order olefins, e.g., C.sub.4 olefins, are important precursors for providing high octane blending components, i.e., C.sub.4 's are precursors to MTBE production and alkylation.
However, when heavy hydrocarbons feedstocks are non-catalytically cracked to olefins it's virtually impossible to achieve the desired co-product ratios to fit market needs, i.e., propylene to ethylene yield ratios are rarely greater than 0.55. Higher ratios are attainable only at low hydrocarbon conversion which represents a significant processing penalty in terms of recycle costs and feed degradation. One well-known non-catalytic cracking process is pyrolysis which typically takes place in the presence of steam at high temperatures. The mechanism by which pyrolysis to olefins is achieved is explained in terms of a free radical mechanism.
At high temperatures, radical initiation takes place by homolysis of a carbon - carbon bond. Once initiated, the free radicals undergo two principal reactions. They are (1) scission at the beta position of the radical and (2) abstraction of a hydrogen, resulting in termination of the reaction.
The scission at the beta position will continue to the point where a methyl radical will be formed at 90 percent frequency. The methyl radical will then abstract a hydrogen atom from another molecule to form methane and another free radical. Ethylene and methane are the principal products from such free radical pyrolysis reactions. Only about 10 percent of the time will a longer radical abstract a hydrogen from a molecule to form C.sub.3 to C.sub.7 paraffins and olefins. Thus, thermal cracking results in high yields of ethylene relative to higher order olefins with the higher order olefins occurring principally as a result of hydrocarbon branching in the initial hydrocarbon feedstock.
One effort at producing increased production of C.sub.3 and higher olefins is directed to subjecting a light hydrocarbon comprising at least one alkane to cracking conditions in the presence of hydrogen sulfide and a solid contact material comprising silica (Kolts, U.S. Pat. No. 4,471,151). The contact material employed, such as silica gel, preferably has a high surface area i.e. at least 50 m .sup.2 /gm. Typical H.sub.2 S concentrations of 0.1 to 10 mole percent based on the alkane feed are employed in the process. It is theorized in Kolts that the improvement in cracking is due to the high surface area material which acts as a catalyst to decompose H.sub.2 S. The result is increased conversion levels with improved selectivity to desired products. However, the improved selectivity to propylene was demonstrated only when cracking n-butane.
The solid contact material employed in Kolts is suitable only for fixed bed operations and not for fluidized bed environments due to its very low mechanical stability. Thus, the solid catalyst of Kolts continues to have the drawbacks of typical catalytic dehydrogenation catalysts designed for fixed beds. These are larger size, diffusion limited catalysts incapable of continuous regeneration in a circulating loop system.
A fluidized catalytic cracking (FCC) unit may also be employed to catalytically produce C.sub.3 and higher compounds. The FCC unit uses acidic cracking catalysts to increase the production of C.sub.3 to C.sub.7 compounds through a carbonium ion mechanism compared to the free radical pyrolysis reaction mechanism. However, the acidic cracking activity of the catalysts, in addition to promoting cracking and isomerization, promotes rapid hydrogen transfer resulting in high yields of paraffins rather than olefins. Further, the nature of the catalytic cracking unit itself favors the shift to paraffins.
The typical definition of residence time in a catalytic cracking operation is the time the feedstock is in contact with the catalyst itself. This definition is acceptable if the temperatures are low such that thermal reactions do not occur to any appreciable extent. However, thermal and catalytic reactions proceed in parallel. While catalyst separation will terminate the catalytic portion of the reaction the thermal reactions (pyrolysis) will continue until the temperature is reduced to a level where the rate of reaction is insignificant (quench). In this situation, the total kinetic residence time can be defined as the time from the introduction of the hydrocarbon into the system to the quenching of the effluent including the separation of the solids from the reaction. Total conversion is thus the summation of the catalytic reaction (time in contact with the catalyst) and the thermal reactions (time at the reaction temperature).
The typical FCC reaction environment has relatively long residence times including time for solids separation (normally greater than one second) and does not include a quench. Cracking takes place at lower temperatures under these longer residence times. Conversion is achieved at these lower temperatures due to the extended contact with the catalyst Thermal reactions are minimized at these lower temperatures thus eliminating the need for quenching the effluent. While increased C.sub.3 and higher compounds are produced in comparison to pyrolysis, the effluent will have a disproportionately high concentration of paraffins due to the increased hydrogen transfer activity. The favored conditions for olefin production, specifically higher temperatures and shorter residence times, are difficult to achieve especially when processing light feedstocks such as LPG and naphthas which require proportionately higher temperatures to initiate and sustain the reaction (either catalytic or thermal).
The above processes all improve the cracking of hydrocarbons to olefins. However, these processes suffer either from high capital and operating costs associated with fixed bed operations and hydrogen sulfide dilution, or result in low yields of the desired olefins. In addition, the use of hydrogen sulfide as a diluent raises environmental and health concerns because of its extremely high toxicity.
It has now been found that the higher order olefins, i.e. propylene, butenes, etc. can be obtained in high yields by the cracking of hydrocarbons in the presence of an acidic cracking catalyst alone or in combination with a noble metal oxide dehydrogenation catalyst in a short residence time fluidized solids cracking environment. This short residence time is achieved by a combination of a low residence time reactor, a very short residence time separation system, and a product quench.
It is therefore an object of the present invention to provide a process in which hydrocarbons can be catalytically cracked to produce olefins and aromatics.
It is another object of the present invention to provide a process for preferentially cracking hydrocarbons to obtain C.sub.3 to C.sub.5 olefins and/or aromatics.
It is another object of the present invention to provide a process in which a hydrocarbon may be cracked to a variety of desired products by altering the catalyst system in the process.
It is a further object of the present invention to provide a reaction system including a quenching step for preferentially cracking hydrocarbons to obtain C.sub.3 to C.sub.5 olefins and/or aromatics while avoiding the thermal degradation of products.